A geological record of changing propagation directions of fault slip events during the growth of a normal fault system

Abstract

The lack of an unambiguous method of determining the propagation direction of slip events on faults over significant time periods limits our understanding of the long-term stability of fault slip propagation directions. A geological means for determining the propagation direction of slip events during the growth of faults is provided by mutually cross-cutting faults and bed-parallel slip-surfaces in the Ptolemais Basin, northern Greece. Bed-parallel slip-surfaces intermittently offset the faults as they grew to form discontinuities on otherwise continuous fault surfaces. Subsequent fault slip events bypassed these discontinuities generating structures that provide a record of their propagation direction. Data collated from 3D fault mapping during lignite mining operations reveal that the slip events on these faults propagated predominantly upwards at low throws. Mixed slip propagation directions at higher fault displacements, up to 50 m, demonstrate that the locations of the points of nucleation of slip events were more distributed over fault surfaces once the faults were established.